This disclosure relates generally to internal combustion engines and, more particularly, to engine valve actuators.
The operation of an internal combustion engine requires, among other things, the timed opening and closing of a plurality of valves. For example, with a typical four-stroke, diesel engine, one of ordinary skill in the art will readily recognize such an engine operates through four distinct strokes of a piston reciprocating through a cylinder, with intake and exhaust valves operating in conjunction with the piston. In an intake stroke, the piston descends through the cylinder while an intake valve is open. The resulting vacuum draws air into the cylinder. In a subsequent compression stroke, the piston reverses direction while the intake valve and an exhaust valve are closed, thereby compressing the air within the cylinder. This is followed by a combustion or power stroke wherein fuel is injected into the compressed air and thereby ignited, with the resulting force pushing the piston again in the descending direction while both the intake and exhaust valves are closed. Finally, the piston reverses direction with the exhaust valve open, thereby pushing the combustion gases out of the cylinder.
In certain variations on the typical diesel or Otto cycle, it is desirable to open or close one of the intake and/or exhaust valves at alternative times. For example, in a compression release braking mode, the exhaust valve is opened as the piston approaches a top dead center position during the compression stroke to, in effect, increase engine braking operation. In so doing the engine cylinders draw in air during the intake stroke, compress the air, and then vent the compressed air out of the exhaust valve near top dead center of the piston.
Another mode of engine operation requiring a typical valve sequencing is known as the Miller cycle. During the Miller cycle, the intake valve is held open during the initial stages of the compression stroke. Such operation reduces the effective compression ratio of the engine and results in a more mechanically efficient power producing engine. Alternatively, the intake valve is closed prior to completion of a normal intake stroke to provide Miller cycle benefits.
One other situation modifying typical valve operation is internal exhaust gas recirculation. One disadvantage of diesel or Otto cycle engine operation is that all of the fuel brought into the cylinder and compressed may not entirely combust. Among other things, this phenomenon may be undesirable due to an unacceptably high level of pollutants, such as nitrous oxide (NOx) and particulates, being released during the exhaust stroke.
Exhaust gas recirculation (hereinafter referred to as xe2x80x9cEGRxe2x80x9d) attempts to curtail such drawbacks of conventional engine operation. With EGR, at least a portion of the exhaust gases is not exhausted to the atmosphere, but rather is introduced back into the engine cylinder to be combusted in subsequent power or combustion strokes of the engine. With typical internal EGR, the exhaust gases are expelled through the exhaust valve and re-introduced to the cylinder through the exhaust valve itself. Such a process requires that the exhaust valve stay open not only through the exhaust stroke, but also on the intake stroke, after the piston reverses direction, thereby creating a vacuum and drawing a portion of the exhaust gases back into the cylinder through the still open exhaust valve.
One of ordinary skill in the art will readily appreciate that a substantial force is required to open the exhaust valve and maintain the valve in an open position as the piston reciprocates through the cylinder toward the top dead center position. A valve actuator employing highly pressurized oil may be used to apply this force to open the exhaust valve.
However, holding an exhaust valve in an open position by a valve actuator employing highly pressurized oil requires, for example, pressurized oil on the order of fifteen hundred to five thousand pounds per square inch (10.34 to 34.4 MPa). The engine or machine in which the engine has been mounted therefore has had to provide a high pressure source or high pressure rail and be able to supply the high pressure oil to the actuator when desired. Such a requirement has, among other things, the disadvantage, at least with respect to Miller cycle and EGR operation, of decreasing the engine efficiency in that the engine must continually direct usable work to the high pressure rail to maintain such pressures even though the high pressure oil is only required for a relatively short duration during the engine operation. Not only is the provision of such pressurized fluid taxing on the efficiency of the engine, but with certain machines the provision of such a high pressure rail is simply not available or desirable.
The present disclosure is directed to overcoming one or more of the problems or disadvantages associated with the prior art.
In accordance with one aspect of the disclosure, an engine valve actuator is provided which comprises an actuator cylinder, an actuator piston reciprocatingly disposed in the cylinder, the actuator piston including a rod associated therewith, a source of pressurized fluid, and a control valve connected in fluid communication with the source of pressurized fluid and the actuator cylinder, the control valve having first and second positions, the first position placing the source of pressurized fluid in fluid communication with the actuator cylinder, the second position maintaining pressurized fluid in the actuator cylinder.
In accordance with another aspect of the disclosure, an engine valve assembly is provided which comprises a valve seat, a valve element adapted to move relative to the valve seat between an open position and a closed position, a mechanically driven actuator adapted to move the valve element to the open position, and a fluidically driven actuator adapted to hold the valve element in the open position.
In accordance with another aspect of the disclosure, an internal combustion engine is provided which comprises a engine cylinder, an engine piston reciprocatingly movable relative to the engine cylinder, an engine valve element disposed in a port connected to the engine cylinder, a valve actuator connected to move the engine valve element, a source of low pressure fluid in fluid communication with the valve actuator, a mechanical linkage mounted proximate the engine valve element and adapted to move the engine valve element, and a control valve adapted to control flow of pressurized fluid from the source of pressurized fluid to the valve actuator.
In accordance with yet another aspect of the disclosure, a method of controlling an engine having at least one valve is provided. The method comprises the steps of moving the valve to an open position, and holding the valve in the open position with an hydraulically locked actuator.